The present invention relates to a control-type brake booster capable of automatically increasing and decreasing braking power by electromagnetically operating an atmospheric valve or vacuum valve.
An example of such a brake booster is disclosed in Japanese Patent Laid-open No. 7-503214 (1995), in which a valve body contains a solenoid mechanism with a member movable by applying electricity to the solenoid of the solenoid mechanism, and a movement of the movable member opens a valve mechanism, which may be an atmospheric valve or vacuum valve, independently of the valve operation by movements of the input rod connected to the brake pedal.
It has been desired that as is the braking power output from a brake booster when its valve mechanism is operated by movements of the input rod connected to the brake pedal, the braking power output from a brake booster having the above-mentioned solenoid mechanism can also be adjusted across a wide range when operated by the activation of the solenoid mechanism.
However, in the above-mentioned conventional brake booster, the solenoid mechanism merely functions to open the valve mechanism to the degree determined by the magnitude of electric current supplied to the solenoid, and no correlation (or proportional relationship) exists between the magnitude of the electric current supplied to the solenoid mechanism and the magnitude of the braking power output from the brake booster.
As a makeshift measure to obtain a desired magnitude of braking power from a brake booster when its solenoid mechanism is operated is to control the operation of the solenoid mechanism based on a feed-back signal indicating the braking power being output from the brake booster, for instance, on an output signal from a master cylinder pressure sensor provided for detecting pressure of braking liquid output from the master cylinder.
However, since an additional sensor is required that functions like the master cylinder pressure sensor to output a signal indicating the braking power being output from the brake booster, the entire system becomes more expensive.
The present invention aims to eliminate the drawbacks in the above-mentioned conventional brake booster. An object of the present invention is to provide a brake booster that, while being simple in construction, can fully adjust the braking power across a wide range when its solenoid mechanism is operated.
According to one aspect of the present invention, there is provided a brake booster wherein an interior of a shell is divided into a constant pressure chamber and a variable pressure chamber by a power piston, and a valve mechanism for controlling supply of operating fluid to the variable pressure chamber by a shifting movement of a plunger connected to an input rod cooperating with a brake pedal is provided within a valve body supported by the power piston, so that a thrust force generated on the power piston by pressure difference between the variable pressure chamber and the constant pressure chamber based on an operation of the valve mechanism is output as a booster output force (thrust output force).
The brake booster further comprises an electromagnetic biasing means having a movable member for operating the valve mechanism independently from an operation of the input rod, and a reaction force mechanism for applying reaction force corresponding to the pressure difference between the variable pressure chamber and the constant pressure chamber to the movable member, whereby the reaction force mechanism applies the reaction force to the movable member so that an increasing rate of the booster output force is in accord with an increasing rate of the reaction force applied to the movable member.
According to another aspect of the present invention, in a brake booster having the electromagnetic biasing means mentioned above and the reaction force mechanism mentioned above, the reaction force mechanism applies the reaction force to the movable member at the same time or with a delay after the booster outputs force.
In the above two aspects of the present invention in which the reaction force corresponding to the pressure difference between the variable pressure chamber and the constant pressure chamber is exerted on the movable member, the booster output force becomes proportional to the magnitude of the electric current supplied to the electromagnetic biasing mechanism. Further, since the reaction force is exerted on the movable member so that the booster output force increasing rate is in accord with the reaction force increasing rate acting on the movable member, or, so that the reaction force is created at the same time or after the booster output force is generated, the booster output force generated is not prevented by the reaction force.
In the present invention, the reaction force mechanism is provided with at least one passage providing communication between a pressure chamber formed at one end of the movable member and the variable pressure chamber. The at least one passage has a limited flow area for limiting air flowing into the pressure chamber. Preferably, the total flow area of the passage is designed 0.5-10 square millimeters.
Limiting the flow area of the passage as above, the timing of the reaction force exerted on the movable member is adjusted properly, and thus a better response of automatic braking can be established.